Diacetyl, a component of artificial butter flavoring, is a potential etiological agent of obliterative bronchiolitis (OB) in employees at microwave popcorn packaging plants; however, the mechanisms of toxicity, specific exposure concentration or duration necessary for OB are unknown. We evaluated the respiratory toxicity of diacetyl in a murine model using several exposure profiles relevant to workplace conditions. Male C57Bl/6 mice (n= 5-6) were exposed to inhaled diacetyl across several concentrations and duration profiles, or by direct oropharyngeal aspiration. Respiratory tract tissues were evaluated by light microscopy as the primary endpoint in all experiments. In selected exposures additional measurements of lung injury were included.[unreadable] Subacute exposure to 200 or 400 ppm diacetyl for 5 days caused deaths, necrotizing rhinitis and inflammation of the large bronchi. Reducing the exposure to 1 hr/day (100, 200, 400 ppm) for 4 weeks caused less nasal toxicity, chronic bronchitis and laryngitis, without bronchiolar lesions. Lymphocytic bronchitis and mild nasal injury were observed after exposures to 1200 ppm (15 minutes, 2x/day, 4 weeks). Subchronic exposures to 100 ppm (6 hr/day, 12 wks) caused lymphocytic bronchitis and bronchiolitis with minimal necrosis in the nasal cavity. Treatment with 400 mg/kg by oropharyngeal aspiration to bypass the nose caused foci of fibrosis with little or no inflammation at the junction of the terminal bronchiole and alveolar duct.[unreadable] Acute exposure to diacetyl caused significant epithelial injury in the nasal cavity and bronchi of mice. Subchronic and intermittent exposures cause lymphocytic bronchiolitis, a potential precursor to OB in humans. Oropharyngeal aspiration of diacetyl caused fibrohistiocytic lesions in bronchioles. Collectively, these results indicate that clinically relevant diacetyl exposures result in a pattern of injury that replicates features of OB.